The success of orthopaedic implants surgically implanted in living bone substantially depends on achieving and maintaining an enduring bond between the confronting surfaces of the implant and the host bone. Surgical procedures for preparing living bone to receive a surgically implanted orthopaedic device have been known for twenty years or more, but the ideal properties of the surface of the orthopaedic implant which confronts the host bone and processes of preparing the implant surface are the subjects of considerable disagreement.
It is generally known that the osseointegration of metallic orthopaedic implants is dependent, in part, on the attachment and spreading of osteoblast-like cells on the surface of the orthopaedic implant. Furthermore, studies suggest that such cells will more readily attach to rough or porous surfaces, as compared to smooth surfaces. To that end, several attempts have been made to provide metallic orthopaedic implants having roughened and/or porous surfaces to aid in the osseointegration of the implants.
For example, U.S. Pat. No. 5,236,459 describes a process for forming an implant surface having “anchoring areas” in which a high-pressure liquid jet is used to remove a portion of the metal from the implant surface. The diameter of the “anchoring areas” can be varied from 0.5 to 1.5 mm.
U.S. Pat. No. 5,307,594 describes another method for forming a textured surface on an orthopaedic implant. This method entails the application of a resilient mask, which contains several openings, to the surface of the implant and then subjecting the implant to high pressure blasting using an erosive blasting media, such as metal oxides particles. While this process can be used to produce implant surfaces having roughened surfaces, particles of the blasting media can become imbedded in the surface of the implant. Furthermore, it is believed that these particles can adversely affect the osseointegration of the orthopaedic implant following implantation.
Each of the above-described methods provides a metallic implant having a roughened surface consisting of surface features that are generally greater than 20 μm in size. While an orthopaedic implant having such surface features may exhibit improved osseointegration as compared to a smooth metallic implant, it is believed that osseointegration will be greatly improved if the implant surface includes smaller surface features (i.e., less than 20 μm in size).
In addition to the mechanical methods of providing a roughened surface described above, various chemical etching methods have been used to texture the surface of orthopaedic implants. For instance, U.S. Pat. No. 5,876,453 describes a two-step process in which a hydrofluoric acid solution (10-50% HF) is used to remove the native oxide surface layer formed on the metallic implant, and a second acid treatment is used to further etch the metal to provide a roughened surface. The second acid treatment utilizes a mixture of two parts sulfuric acid (96% by weight H2SO4) and one part hydrochloric acid (37% by weight HCl). While this process and similar chemical etching processes are capable of producing roughened metallic implants having surface features less than 1 μm in size, such aggressive acid solutions often remove a relatively large amount (several hundred microns) of the metal from the surface of the implant. It accordingly becomes difficult to use such aggressive solutions without undermining the structural integrity of orthopaedic implants of smaller size.
Another method to enhance achieving and maintaining the desired bond between an implant and the host bone has been to apply metallic beads to the surface of the implant. Then, the beads are sintered to bond the beads together and to the surface of the implant. This method, described in U.S. Pat. No. 3,855,638, produces a porous surface on the metallic implant which consists of interstitial pores having an average size of approximately 20 to 200 μm uniformly distributed throughout the surface of the implant.
Implants with such a porous surface represent a widely used and effective approach. However, using mechanical methods to further roughen the porous surface of such implants is particularly problematic, as the porous surface is difficult to clean so as to satisfactorily remove particles used in the grit blasting process. Such particles can tend to collect in the interstices between adjoining beads where removal can become exceedingly difficult.
Even further, the use of previously known chemical etching methods would seriously undermine the structural integrity of orthopaedic implants of this type. Such loss in integrity can occur due to impairing and/or destroying the bonds between adjacent beads, and even dissolution and destruction of individual beads, as well as the diminishing of the bond between an individual bead and the surface of the metallic implant substrate or body.
In many applications, it is desired to provide the implant with a bioactive coating comprising, as one example, calcium phosphate materials, so as to further promote and enhance the growth of bone and/or apposition of bone at the surface of the implant after implantation. It is thus important that any surface roughening process be compatible with any desired bioactive coating and to provide adequate adherence between the bioactive coating and the metallic implant.
A need therefore exists for a method of producing a roughened surface so as to enhance osseointegration, on implants having a complex surface geometry. A need also exists for a method of producing a roughened having metallic beads without significantly and adversely affecting the structural integrity of such implants. Furthermore, a need exists for a method of producing a suitably roughened surface on a wide range of implants which can be affectively and reliably carried out and for such a method that is fully compatible with the formation of bioactive coatings.
The invention provides such a metallic orthopaedic implant and a process for producing the same. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.